Effect Of Sm,La And Ca On Microstructures And Mechanical Properties Of Magnesium And Its Alloys

Magnesium alloys are the important choice of lightweight materials for transportations,while magnesium alloys still have some problems as the the main load-bearing structural and heat-resistant structural materials,such as low strength,plasticity and mechanical properties.Magnesium alloys with high strength and toughness are mostly aqcuired by high alloying of heavy rare earth elements at present,which can improve mechanical properties of magnesium alloys but also increase their costs,so that their applications in civil fields are still limited.In recent years,the application of relatively cheap light rare earth elements in the development of new magnesium alloys has attracted more and more attentions.As one of the cheapest light rare earth elements,Sm element is not fully utilized,and is rarely used in practical applications of magnesium alloys.This paper focuses on the application and development of Sm in magnesium alloys,chooses industrial pure magnesium and the most widely used AZ series magnesium alloys(AZ31,AZ61,AZ80)as the based alloys,and intends to study the effects of Sm,La and Ca on micro structures and mechanical properties of magnesium and its alloys by adding relatively cheap rare earth elements Sm,La and cheap alkaline earth element Ca.The microscopic mechanism is discussed preliminarily.The main findings are as follows:(1)After adding 0.5%Zn and 0.2%Mn(mass fraction,same as below)into industrial pure magnesium,and adding 0.2%Sm+0.3%La,0.2%Sm+0.4%Ca or 0.3%La+0.4%Ca,the microstructure of as-cast pure magnesium is significantly refined,and the trial magnesium alloy with 0.2%Sm+0.4%Ca additions has the best refined as-cast microstructure,while the trial magnesium alloy with 0.3%La+0.4%Ca additions has the best refined as-extrued microstructure.The above trial magnesium alloys exhibit high plasticity at room temperature,and their elongations reach 20%-30%.The strengths of trial magnesium alloy with 0.2%Sm+0.3%La additions are close to that of AZ31 alloy.While the other two trial magnesium alloys have higher strength at 150℃;(2)Adding 0.3%La to Mg-0.5Zn-0.2Mn alloy can significantly refine its as-extruded microstructure and remarkably improve its strength and plasticity both at room temperature and 150℃.This is related to the formation of Mg12La strengthening phases with higher melting point in Mg-0.5Zn-0.2Mn-0.3La alloy.After further adding 0.2%Sm into Mg-0.5Zn-0.2Mn0.3La alloy,the as-extruded microstructure is significant refined and the plasticity at 150℃ is remarkably improved,of which the elongation is close to 70%;(3)Adding 0.3%La can significantly refine the as-extruded microstructure of AZ31 alloy,and remarkably improve its strength at room temperature,while it has little effect on AZ61 and AZ80 alloys.Adding 0.3%La can remarkably improve the plasticity of AZ31 and AZ61 alloys at 150℃,and their elongations reach 60%,while strengths of the above three AZ alloys decrease slightly.After adding 0.2%Sm to AZ31-0.3La,AZ61-0.3La and AZ80-0.3La alloys,the elongation of AZ31-0.3La-0.2Sm alloy reaches 25%,but adding 0.2%Sm has little effect on strengths and plasticity of the above three alloys at 150℃.With the increase of Sm additons in AZ31-0.3La-0.2Sm,the yield strength at room temperature increases gradually,and when Sm content is not higher than 1%,its elongation at room temperature can be stabilized at 20%～25%;(4)Adding 0.3%Ca or 1.3%Ca to as-extruded AZ31 alloy can obviously improve strengths at room temperature and 150 ℃,while its mechanical properties are not further improved after further adding Sm,La.Adding 1.5%Ca,0.5%Sm or 0.3%La separately could refine the microstructure of as-cast and as-extruded AZ91 alloy,and the elongation of the above three alloys are stable to 23%-26%.Adding 0.5%Sm or 0.3%La could also improve mechanical properties of AZ91 alloy at 150℃.Although the microstructure of as-cast and asextruded AZ91 alloys is refined obviously with combined additions of the above three elements,its strengths change slightly and plasticity slightly decreases at 150℃.(5)Adding 0.4%Ca,0.2%-0.3%Sm or 0.3%La has little effect on basal texture of AZ alloys.After adding 0.3%La to AZ31 alloy,the yield behavior is dominated by tensile twinning,and after 0.2%Sm is added further,the yield behavior is changed to be dominated by slip,and the basal texture of AZ31 alloy is significantly strengthened when Sm addition is increased to 1.7%.